Technological developments in the telecommunications industry, linked with developments in integrated electronics and in computing, have led to very rapid development of services to users and to the facilities available to personnel running exchanges. Increased component reliability and developing test means have led people to seek for security systems capable of increasing the quality of service and of reducing the number of urgent interventions required and also the frequency of normal maintenance interventions.
An effective security system requires rapid-acting detection means for detecting faults and responding thereto, for example by switching over to a spare member. The system also requires means for accurately localising the members affected by the fault and means for enabling maintenance personnel to act efficiently. These include display interfaces and means for assisting repair.
In exchanges controlled by a central processor, a technique which was developed during the 70's, system security is likewise centralised: the processor is duplicated, auxiliary members and members of the switching network are duplicated, and specific means are provided for testing the telephone equipment under processor control and for indicating the results.
Nowadays it is is possible to benefit from the advantages of distributed control, this is most particularly due to the development of microprocessors and their software.
Unfortunately, the advantages and flexibility in use and the modularity of a distributed control system tend to be in conflict with requirements for a reliable security system.
Thus, in a distributed control exchange having several thousands of lines, it is necessary to have two levels of microprocessor control: the first level in which the processor supervises a plurality (up to a few tens) of terminals and a second level concerning hundreds or thousands of terminals, e.g. for controlling switching, charging or maintenance. Now a modular exchange necessarily comprises some number of blocks or groups of equipments, and the second level of control may either be integrated with these groups or else it may be outside them, in which case a second level processor is not necessarily rigidly associated with any particular group.
When the second level is integrated with these groups, a security system can readily be organised on a local level. A system of this type is described, for example, in published French patent application No. 2 420 260 entitled "Distributed control digital switching system" in the name of International Standard Electric Corporation. A drawback with this solution comes from the fact that the second level processors need to be duplicated.
A two-level system of control in which the second level is not integrated with the modules is described in an article by Shimasaki entitled "A versatile digital switching system for central office NEAX 61" (proceedings of the ISS Paris colloquium 79-7 on the May 11, 1979) volume 2 pages 688 to 695. This system provides distributed security in association with centralised maintenance. However, such a system still has drawbacks in that system security requires specific links between the two levels and also in that the second level processors are not interchangeable which means that they must be duplicated.
Another two-level system in which the second level comprises groups of interchangeable processors which are connected to one another and to the processors on the first level by semaphore channel data links is described in the Applicants' French patent application No. 81 068 07 entitled "A distributed control time division exchange".
Preferred embodiments of the present invention provide a security system for application to an exchange of the last mentioned type in which the security system does not detract from the modularity and flexiblity of the exchange.